1. Field of the Invention
The invention pertains to a method of making an optical device comprising a substrate on which are integrated a layered optical waveguide component comprising a polymeric guiding layer sandwiched between two deflection layers of a lower refractive index than the guiding layer, and optical fibre ends, the optical fibre ends being positioned in grooves.
2. Description of the Related Art
Optical fibre ends attached to the integrated optic device are usually referred to as "pigtails," and the process of providing a waveguide component with such pigtails is referred to as "pigtailing." The invention also pertains to the pigtailed optical waveguide devices so obtainable and to the free-standing, flexible waveguide sheets used in making them.
In general, flexible waveguides are known. e.g., JP 04/232906 discloses a flexible multilayer light guiding sheet suitable for use in signal transmission, e.g., with light splitting such as optical data linking for cars.
JP 05/281428 pertains to a flexible optical interconnection board comprising a flexible optical waveguide mounted onto a base plate that can have various shapes (e.g. curved).
From JP 04/274402 it is known to provide a flexible waveguide by coating a polyamic acid solution onto a substrate, irradiating it so as to produce a precured film, removing said film from the substrate, and curing it so as to form a polyimide waveguide. A polyimide film having a lower index of refraction than the polyimide waveguide film is used to coat at least part of an upper layer and a lower layer of the waveguide film.
It is known to produce polymeric flexible waveguides having a refractive index pattern, see JP 52/138 146 which teaches forming a polymeric film, diffusing a monomer in it, and polymerizing the monomer in selected areas. Comparable techniques have been disclosed in JP 78/026813 and JP 54/088144.
From U.S. Pat. No. 4,496,215 it is known to provide an optical interconnection device comprising straight and curved segments, in which the optical interconnection is in the form of fibres (laid in a layered flexible structure).
These references do not specifically address the problem of pigtailing polymeric optical waveguide components. This continues to be one of the principal challenges when making optical waveguide devices. Making a proper connection with a minimum loss of light (coupling loss) between the guiding layer of the waveguide component and the core of the optical fibre is a cumbersome, and generally expensive process step. This especially holds for coupling with single mode optical fibres.
It is known to make use of substrates (usually of silicon) having V-shaped grooves in which fibre ends can be placed. The V-shape of the grooves allows the fibres to be properly aligned vertically (i.e., in respect of the height of the guiding layer) as well as laterally (i.e., in respect of the width of waveguiding channels made in the guiding layer). After the fibres have been placed in the V-grooves they are usually fixed using glue, solder, or the like.
A method as indicated above, employing such a V-grooved substrate, is known from GB 2 000 877. The method disclosed bonding the end-portions of a plurality of optical fibres to V-shaped grooves provided on a transparent substrate by an adhesive; casting a polymer solution onto the fixed optical lead fibres-containing substrate, and evaporating the solvent. Thus, the optical lead fibres are embedded in a polymer layer. Said polymer layer, which serves as the guiding layer of the optical waveguide component, contains, int.al., a photopolymerizable monomer. By selectively activating this monomer, the refractive index of the guiding layer can be selectively decreased. Thus, waveguide channels can be formed in the guiding layer by irradiating the surrounding material. These channels can be made to be aligned with the positions of the embedded ends of the optical fibres. On the guiding layer a low refractive index coating is applied as a top deflection layer. The substrate serves as the lower deflection layer.
The method disclosed in GB 2 000 877 has several drawbacks. In part, these are associated with the use of the substrate as the lower deflection layer. One disadvantage thereof is that the disclosed method cannot be applied to make electro-optically active waveguides. E/O-active, or NLO materials, are known. In such materials non-linear charge polarization occurs under the influence of an external electric field. Non-linear electric polarization may give rise to several optically non-linear phenomena such as frequency doubling and Pockels effect. Obtaining the desired NLO effect in polymeric materials macroscopically requires that first the groups present in such a material, mostly hyperpolarizable side groups, be aligned (poled). Such poling is usually effected by exposing the polymeric material to electric (dc) voltage, the so-called poling field, with such heating as will render the polymeric chains sufficiently mobile for orientation. Hence, if a polable e/o material is used in the guiding layer, it is a requirement for poling to occur that the guiding layer can be exposed to an electric field. To this end, two electrodes are needed, one of which is usually applied on top of the layered waveguiding structure, while the other is usually applied at the bottom, i.e., in between the substrate and the lower deflection layer. Providing such a bottom electrode is not envisaged in the method of GB 2 000 877.
Another disadvantage of using the V-grooved substrate as the lower deflection layer is that the refractive index of such a lower deflection layer cannot be tailored to that of the guiding layer and the top deflection layer. Since the difference in refractive indices (the refractive index contrast) of the layers determines the efficiency with which light from an optical fibre can be coupled into the waveguide and viceversa, it is a serious design limitation if the refractive index of any one of the layers cannot be freely chosen. This particularly holds for the deflection layers, since, depending on the nature of the device, the requirements applicable to the guiding layer polymer frequently are more stringent than those applicable to the cladding layers. E.g., in the case of electro-optically active guiding layers, the choice of the guiding layer polymer will be determined more by its e/o coefficient than by other properties such as refractive index. Or, if it is desired to form channels in the guiding layer by means of "bleaching" (a photophysical change of refractive index such as disclosed in EP 358 476), the guiding layer polymer may be chosen for its bleaching capacity more than anything else.
The method of GB 2 000 877 cannot be simply replaced by a method in which a separate lower deflection layer is applied on the substrate. To begin with, this would undo the very fibre-waveguide alignment as taught in the disclosure. Further, if the lower deflection layer is polymeric, which is highly desirable in respect of tailoring the refractive index and compatibility with the polymeric guiding layer, the problem applies that the V-grooves will fill up with the deflection layer polymer, leading either to the fibre cores being severely misaligned with the light guiding channels of the polymeric waveguide or unflatness and thickness nonuniformity for the waveguide structure, or both.
Another known method for connecting optical fibres with waveguide devices employing V-grooved Si is known from IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol.13(4), 1990, pages 780-786. The method involves bonding a first substrate, a portion of which carries optical fibres in V-grooves, with a second substrate carrying an integrated optical circuit, the fibres and the circuit eventually being positioned adjacent to each other. The two substrates face away from one another, and the bonding is achieved by means of solder bumps. This so-called flip-chip solder bump bonding is a self aligning process, the alignment being achieved through the action of surface tension forces during solder reflow. This method has several drawbacks in respect of pigtailing polymeric waveguide components. The required melting and flowing of solder generally requires temperatures that are too high to be used with polymers. A typical temperature being 200.degree. C., this will be above the glass transition of many types of electro-optically active polymers, which will lead to these polymers losing their activity. Further, the flux (organic acid that prevents oxidation of the solder) that as a rule is dispensed on the solder during reflow, is likely to attack the polymer. Further the method has the drawback of the final product comprising no less than three different substrates: the two substrates mentioned, i.e., one carrying the integrated optical circuit and the other carrying the fibres in V-grooves, together need be supported by a third substrate, a so-called motherboard.